1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method which performs exposure by irradiating a substrate with exposure light via a projection optical system and liquid.
2. Description of Related Art
Semiconductor devices and liquid crystal displaying devices are manufactured by transferring a pattern formed on a mask onto a photosensitive substrate, in other words, through a photolithography technique. An exposure apparatus which is used in the photolithography processing, includes a mask stage which holds a mask, and a substrate stage which holds a substrate. The exposure apparatus transfers the mask pattern onto the substrate via a projection optical system, while successively moving the mask stage and the substrate stage.
Recently, in order to realize higher integration of the device pattern, there has been demand for higher resolution of the projection optical system. When the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. When the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in the exposure apparatus has shortened as years go by, and the numerical apertures of projection optical systems have also increased. Further, the currently mainstream exposure wavelength is the 248 nm of an KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer laser is also being commercialized In addition, in addition to resolution, the depth of focus (DOF) is important when exposure is performed. The resolution R and the depth of focus δ are respectively given by the following formulas.R=k1·λ/NA  (1)Δ=±k2·λ/NA2  (2)
It is note that λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1, k2 are process coefficients. According to formula (1) and formula (2), in order to increase the resolution R, if the exposure wavelength is shortened and the numerical aperture NA increases, it is known that the depth of focus δ becomes narrower.
If the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there occurs the possibility that the focus margin during the exposure operation will be insufficient. As a method for essentially shortening the exposure wavelength and expanding the depth of focus, for example, in International Publication No. WO 99/49504, a liquid immersion method is disclosed. This liquid immersion method is designed to, by filling the space between a lower surface of the projection optical system and the substrate surface with liquid such as water, organic solvent or the like, form a liquid immersion area and by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid, and is normally about 1.2 to 1.6) of that in the air, improve resolution and, at the same time, extend the depth of focus by approximately n times.
Now, in order to maintain exposure accuracy in the exposing process following the liquid immersion method, and to prevent deterioration of the pattern exposed onto the substrate, it is important to supply and recovery the liquid smoothly.
Specially, if the liquid cannot be recovered smoothly, the remaining liquid which cannot be recovered flows out, and surrounding mechanical parts rust. Further, the environment around the substrate (humidity, etc.) changes, and thus the refractive index on an optical path of a detection light of a light interferometer used to measure the position of the stage changes. Therefore, various measuring operations relative to the exposing process may be affected. As a result, exposure accuracy is lowered.
In addition, if a liquid supply device and/or a recovery device vibrate, the pattern image projected onto the substrate may deteriorate due to the vibration, and a measuring error of the interferometer which monitors the position of the substrate stage holding the substrate may be caused. The present invention was made by taking such circumstances into consideration, and has as an object to provide an exposure apparatus and a device manufacturing method, which can maintain exposure accuracy in an exposing process which is performed by irradiating a substrate with exposure light via a projection optical system and a liquid.